Extrusion dies have been found to be useful in forming cellular or honeycomb ceramic substrates for use in catalytic converters utilized in the exhaust system of internal combustion engines. In order for such converters to function efficiently, it is necessary that the cells provide a substantially large surface area for catalytic material to react with the exhaust gases, and that the cell walls have a substantially thin cross-sectional dimension so as to provide a substantially large open frontal area and thereby reduce back pressure within the exhaust system. However, the thin walled structure must have sufficient mechanical and thermal integrity so as to withstand normal automotive impact and thermal requirements.
In order to provide increased surface area within the honeycomb structure so as to enhance catalytic activity, the number of cells has been increased from about 175 per sq. in. to 400 or more cells per sq. in., whereas the wall thickness between the cells has been reduced from about 10 mils to 6 mils or less. Accordingly, since the present invention is directed toward the extrusion of honeycomb structures having up to about 600 cells per sq. in. with a wall thickness of down to about 4 mils, the manufacture of the extrusion dies necessary for producing such honeycomb structures has required a variety of technological advances.
As pointed out in U.S. Pat. No. 3,905,743, extrusion dies may be formed in unitary die blocks by utilizing conventional machining and cutting techniques, electric discharge machining, or chemical machining. In addition, a plurality of machined or cut stacked plates are disclosed in such patent for forming laminated dies useful in extruding honeycomb structures. Further, U.S. Pat. No. 3,923,444 discloses a laminated extrusion die for forming honeycomb structures wherein a plurality of elongated extrusion plates are stacked upon one another and clamped together to form an extrusion die. However, it has been found that when forming honeycomb structures having a wall thickness of about 0.006" or less with the use of laminated die plates, the batch material had a tendency to flow longitudinally through the die with insufficient lateral flow to produce good knitting of the interconnected cellular walls of the substrate. In other words, the discharge slots which are formed longitudinally between the laminated plates, have a tendency to function as a continuation of the feed slots and are provided with adequate flow, whereas the discharge slots formed transversely across such plates, do not receive sufficient flow of the batch material to form complete knitting, and thus a continuous cellular mass is not provided prior to being discharged from such slots.
The present invention has overcome the batch flow problems encountered with laminated dies when forming cellular substrates having relatively thin wall thicknesses of about 0.006" and below. A baffle plate is provided between adjacent die blades so as to adjust the flow impedance of the batch material within the discharge slots between the teeth of adjacent plates, to be similar to the impedance to batch flow which is encountered within the discharge slots between adjacent teeth formed in each die blade. Thus, the impedance to the batch flow within the various discharge slots is equalized, and a continuous cellular mass is formed within the die prior to being discharged longitudinally therefrom.